MASSIVE levels of potentially killer E.coli have led to a "don't punt here" warning signs on the river near Cambridge University.
The harmful bacteria on the River Cam at Sheep's Green - where students pole their punts - is six times above the approved level.
It was declared an official safe spot for swimmers and boat users, weeks before.
Some punt crews have claimed to become violently ill after coming into contact with the river water.
To the embarrassment of the authorities, it was named as one of the government's new safe bathing water sites on 13 May.
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The Environment Agency revealed today that the E. coli in the river is around six times higher than what is considered acceptable.
Terry Macalister, from the Friends of the Cam group, said "We were opposed to the whole idea that this should be called a designated bathing area because we feared that could only encourage people to swim here.
"Now we know absolutely for certain that there's a health danger from actually swimming here.
"I used to swim here every day, it was one of my great pleasures in the summer.
"But a number of groups have been taking tests and clearly began to show that the river was heavily polluted, so I've stopped swimming here."
Punting, a tradition going back centuries, plays a vital role in Cambridge's tourism industry and operators fear water pollution will put off visitors.
Their staff are encouraged to use sanitisers and customers are urged not to put their hands into the water.
Campaigners blame the pollution in sewage coming from an Anglian Water plant.
"No doubt some of it is agricultural run-off from animals further up the river, but undoubtedly some of it is sewage," said Mr Macalister.
Anglian Water said: "We will be carrying out a source apportionment study in 2025 to determine what is contributing to the poor water quality at Sheep's Green, which will help us and other stakeholders on the River Cam to funnel any investment towards the most effective solutions."
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The testing comes after Sheep's Green was among 27 sites in England to be given designated bathing status last month.
Under the designation, the Environment Agency will monitor the site during the bathing season, which runs from 15 May to 30 September.
The first test - a 'pre-season' test - was conducted at Sheep's Green on 8 May. It found 2,700 E. coli colonies per 100ml and 240 intestinal Enterococci colonies per 100ml.
A second test, conducted on 15 May, found the levels of E. coli colonies had dropped to 2,400 per 100ml, but the level of intestinal Enterococci colonies per 100ml had risen to 460.
A test on 24 May found 5,900 E, coli colonies per 100ml and 2,700 intestinal Enterococci colonies per 100ml.
The Environment Agency takes up to 20 water samples at each of England's designated bathing sites during the water bathing season.
The tests look for bacteria that indicates whether there is faecal matter in the water.
Bathing waters can be classified as "excellent", "good", "sufficient" or "poor".
The agency will assess whether action is needed to cut pollution levels.
How concerning is this outbreak?
Dr Richard Anderson, head of learning and development at the food hygiene experts High-Speed Training, said: "The news that food poisoning cases are soaring is deeply concerning.
"A combination of looser checks on imported meat entering the country, combined with budget cuts for local authorities and environmental health standards agencies, has created a food environment in the UK that poses a higher risk to consumers.
"Because of this, it’s important that Brits understand what food types class as high risk, and how to safely prepare and cook them in the home.
The most common forms of food poisoning in the UK come from raw or undercooked foods.
"E.coli is a potentially fatal infection, particularly in older people. You can prevent it by thoroughly cooking your meat," he added.
Bacteriologist and food safety expert Prof Nicola Holden said: “Any outbreak of STEC is a concern because it is a serious, notifiable pathogen that can cause severe and sometimes fatal disease."
The bug tends to more seriously affect young children, she explained, with symptoms ranging from diarrhoea and vomiting to bloody diarrhoea and haemolytic uremic syndrome (HUS) - a rare kidney condition.
As to whether we can expect to see more people fall ill with STEC, Prof Holden said: “It depends on how long the outbreak has been going for.
"UKHSA have said they expect the numbers to increase as they continue with detection."
She said case numbers will depend on the source of infections.
The outbreak might last longer if it's related to a perishable product, but it may be "more prolonged" if it's tied to a product with a longer shelf life.
“UKHSA will be working with colleagues from different public bodies, including our excellent food standards agencies to identify the source," Prof Holden added.
It comes after people suffering from diarrhoea and vomiting were told to stay at home.
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